Apparatus and method for continuous removal of oxides from metal

ABSTRACT

Oxides on the surfaces of metal are reduced by directing reducing gases at them in a forceful and turbulent manner in an enclosure. The oxide-bearing surface is heated to at least 900 degrees F.

TECHNICAL FIELD

[0001] This invention relates to the reduction and removal of oxidesfrom the surface of metal. The metal containing surface oxides is passedinto or through an enclosure, continuously, intermittently, orbatchwise, in which it is heated and contacted with reducing gas.

BACKGROUND OF THE INVENTION

[0002] Newly formed metal strip, rod, and the like tends to developoxides on its surface which must be removed before further processing.In the steel industry, this oxide layer is called mill scale. Mill scaleis almost universally removed by acid pickling.

[0003] Hydrogen and other reducing gases such as carbon monoxide havebeen used for the reduction of oxides in ores, where they aresubstantially consumed within a reducing furnace or vessel. Hydrogen isreadily burned and can cause explosions under certain circumstances, andcarbon monoxide is poisonous and generally considered dangerous unlessconfined and reacted in a vessel of the type generally contemplated inore reduction. Moreover, steel strip and many other metal products madecontinuously move at a rapid pace, increasing the difficulty ofconducting the oxide removal process with gases within the timeconstraints normally imposed. Thus, while the elementary chemicalprinciples of oxide removal and/or reduction by reducing gases areknown, an acceptable continuous surface oxide reduction system employingreducing gases has not been forthcoming in the art.

SUMMARY OF THE INVENTION

[0004] My process and apparatus provide for three stages or zones forthe processing of the moving metal, which may be any metal having oxideon its surface, in any commercially common shape, such as strip or rod.The three basic stages are heating, reducing, and cooling. All threesteps take place within an enclosure of the type to be described in moredetail below, and under the conditions to be described in more detailbelow. Heating in the heating zone is accomplished by a combination of aheating element or device to be described below and post-combustion ofunreacted reducing gas. Reduction of the oxide scale in the reductionzone is accomplished by assuring a turbulent and/or vigorous applicationof reducing gas to the surface of the metal, preferably in the presenceof elemental carbon; cooling of the metal in the cooling zone prior toits exit from the enclosure is accomplished by the introduction of inertgas along with the unheated reducing gas to contact the reduced surfaceof the metal just prior to its exit from the enclosure. The metalsurface should be cooled to a temperature at which reoxidation isunlikely to occur; in the case of steel strip, this is 500° F. or lower.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a more or less diagrammatic side sectional view of apreferred configuration of the enclosure including all three zonesincluded in my invention, as applied to steel strip.

[0006]FIG. 2 is an overhead view from within the same enclosure.

[0007]FIG. 3 shows a preferred device for distributing carbon on thestrip surface.

DETAILED DESCRIPTION OF THE INVENTION

[0008] It is known that the oxide layer on steel strip may containFe₂O₃, Fe₃O₄, and/or FeO, or various ratios of the three oxide formsdepending on the conditions in which the product is made and conductedto the next processing stage. Fe₃O₄ may pass through the Fe₂O₃ stagebefore it is further reduced to FeO and then completely reduced to iron.Where hydrogen is the reducing agent, water is produced; where carbon isthe reducing agent, carbon monoxide is first produced, and where carbonmonoxide is the reducing agent, carbon dioxide results. My inventioncontemplates the use of either hydrogen or carbon monoxide, or any othercommercially feasible reducing gas, in the absence of or together withelementary carbon as a supplementary reductant.

[0009] Further, the hydrogen may be manufactured within the enclosure orin its immediate vicinity. Examples of the manufacture of hydrogeninclude known processes for accomplishing the dissociation of methane,and the combustion of methane or other hydrocarbons in such a way as toproduce excess hydrogen.

[0010]FIG. 1 illustrates the invention applied to steel strip 1 fromwhich mill scale, or a layer of oxide, must be removed. Steel strip 1 iscaused to pass into enclosure 2 in the direction, as depicted, from leftto right. It may be held in enclosure 2 for a period of time or movingat a speed up to as fast as 2000 feet per minute. The strip 1 may bepreheated before entering enclosure 2, but is heated within enclosure 2by heating elements 3, preferably radiant heaters, to ensure that thetemperature of its surfaces is at least 900° F. by the time it leavesthe heating zone, which is designated by the numeral 4.

[0011] At the entrance of the strip 1 to the enclosure 2 is a flame 13and a flue 9 for conducting exhaust gases out of the system. The heatingof strip 1 is assisted by the post-combustion of the unconsumed reducinggases by air optionally introduced through inlets 14 in the heating zone4. Introduction of the air through inlets 14 will cause immediatecombustion of whatever reducing gas, usually hydrogen, remains in theatmosphere moving from right to left, as depicted. Preferably the flowof air will be directed at the strip so as to ensure the most efficientuse of the thermal energy generated by the combustion, that is, to heatthe strip. The action of the flame 13 creates a draft continuouslymoving gases from right to left, as depicted—from the enclosure stripexit 15 to the strip entrance 16, thus providing a constantcountercurrent contact of gas to the strip.

[0012] The strip 1, supported by rolls 5 and 6, is then passed intoreducing zone 7. Rolls 5 and 6 may be replaced by any suitable support,and also may be replaced by graphite or carbon blocks of a consistencyso that a thin film of elemental carbon is deposited or rubbed onto thestrip surface, preferably both the top side and the under side. Reducinggas 11, usually hydrogen, is continuously introduced through smallapertures 17 (see FIG. 2) in manifolds 10, and directed, preferably at aslight angle of 5-30 degrees, in the direction of the oncoming strip 1at a velocity to create turbulence on impact with the strip 1. Wherecarbon is deposited on the strip, the deposition preferably occurs inthe upstream half of the reducing zone 7, so there will be time for itto react with the oxides on the surface of strip 1. This zone is calledthe reducing zone because a large part of the reduction of the oxidesoccurs in this zone, but it should be understood that some oxide may bereduced in the heating zone 4 due to the continued presence there of atleast some reducing gas, and in the cooling zone 8 in part because ofthe continued presence of reducing gas carried into the cooling zone 8by strip 1. In the reducing zone 7, the temperature of the surfaces ofthe strip in maintained at the temperature necessary for the reducingreaction to take place. In the case of steel strip, this is above 900°F.

[0013] Moving on, the strip 1 passes into the cooling zone 8. In coolingzone 8, the strip 1 is caused to cool by the introduction of newreducing gases through manifolds 10. The reducing gases introducedseparately through manifolds 10 may be mixed with inert gases introducedthrough separate inlets 21 or premixed with the reducing gases.Introduction of inert gases here will minimize the possibility of mixingair with the reducing gases. When used, inert gases may be mixed withthe reducing gas in volume ratios of from 1:99 to 99:1. The strip thenpasses out of enclosure 2 through fabric curtain 12 and may be coiled orfurther processed in a hot or cold rolling mill, a slitting station, agalvanizing line, or it may be oiled, otherwise processed, or simplycoiled.

[0014]FIG. 2 illustrates the parts of enclosure 2 from above heatingelements 3 and manifolds 10. Strip 1 is underneath heating elements 3and manifolds 10. Manifolds 10 are seen to have a plurality of gasapertures 17 for releasing gas. These are on the underside of themanifolds 10 and aimed so the reducing gas may be directed with forcetoward the strip 1, preferably in the direction from which the strip 1is traveling. Heating elements 3 have electrical connections 16. Notethat divider 18 appears only on the top side of strip 1 (see FIG. 1);dividers 19 and 20 are above and below the strip 1. Preferably thereducing gas manifolds 10 have one or two lengths 28 within enclosure 2before releasing gas through apertures 17, so the gas can be partiallypreheated before being released.

[0015]FIG. 3 is an optional device for depositing elemental carbon onboth sides of strip 1. The device includes carbon blocks 23 and 24secured to bases 25 and 26, which in turn are connected to pneumaticcylinder 27 made to urge the carbon blocks 23 and 24 toward strip 1. Thecarbon blocks 23 and 24 may be made of graphite, anode pitch, or anyother convenient composition substantially of carbon which will deposita thin film of carbon on the strip as it passes between the blocks 23and 24. Alternatively, only one block may be used; in either case thecarbon blocks may to some extent replace or supplement the supportingfunction of rolls 5 and 6 (FIG. 1).

[0016] The following guidelines may be used for the treatment of steelstrip by my invention, although it should be understood that myinvention is applicable to other metals having oxides on their surfaces.

[0017] Typically, steel strip will have an oxide layer about 0.009 inchthick, commonly from 0.005 to 0.015 inch, and contain about 1 mole toabout 1400 moles of oxygen per square meter of surface. Thus, about 1.1moles to about 1400 moles of hydrogen, will be required for completereduction of the oxides. However, it is known that the microstructure ofthe scale shows numerous small crevices between adherent particles ofiron oxide, and a significant portion of the oxide is effectivelyundermined and loosened by the effect of the reducing fluid. Myinvention therefore requires that the reducing gas is contacted with theoxide layer in a vigorous, turbulent manner to assure the continuousreplenishment of reactants to the metal/oxide surface and continuousconvection of the reaction products, i.e. especially water, away fromthe gas/solid interface. This vigorous, turbulent contacting to enhancethe gas phase mass transfer is preferably accomplished by introducingthe gas through ports directed toward the surface from which the oxideis to be removed. Because of the undermining and loosening effectsmentioned above, it is not necessary for every atom of oxygen to reactwith a reducing gas; as a significant portion of the oxide will besufficiently loosened and/or undermined that it can be easily removedmechanically, such as by brushing; in addition, the turbulent action ofblowing the reducing gas on the surface of the strip in the stripcooling zone 8 will loosen and remove some of the oxide particles.

[0018] To further enhance the reducing reaction in the reducing zone,reducing gas may be introduced directly to the reducing zone after firstbeing preheated. Because gas in the cooling zone is employed partly tocool the strip, the gas introduced there is not to be preheated.Preheating of gas for introduction to the reducing zone may desirably beto a temperature of 900 to 2000° F., and can be accomplished at leastpartially by directing the fresh reducing gas through extra lengths 28of manifolds 10 within enclosure 2, where it will pick up heat energyfrom the environment. Prior to passing into such pipes within theenclosure, the gas may be partially preheated by any suitable means.

[0019] Only the surface need be heated to the desirable reductionreaction temperature. Suitable devices for heating are radiant tubes,induction coils, and gas burners. By heating of the surface, I mean theoxide layer, which may be from to 0.005 inch thick to 0.01 inch thick,on steel strip, and seldom more than 0.015 inch. Thus, temperatures of900° F. need not extend to a depth of more than 0.017 inch and, in mostcases, 0.015 inch will be sufficient.

[0020] In addition to the heating methods and means mentioned above,heating of the reducing gas may be accomplished by passing it throughpassages in heated carbon blocks.

[0021] It will be noted that my invention contemplates a use of thereducing gases to a such degree of efficiency that no recycling isnecessary. Recycling of the exhausted reducing gas stream would requireremoval of the chief reduction product, water, from the gas to berecycled, which is very difficult to do to the extent necessary.Likewise, it would mean cooling the recycled reducing gas, thus settingup a continuous process of heating and cooling of the reducing gas.Rather, my invention contemplates the efficient use of the reducing gasin enclosure 2 by inducing turbulence and direction of the gas onto thesurface of the metal to assure continuing contact and replacement of gasand reduction products on the surface. Preferably at least 50%, and mostpreferably at least 90%, of the reducing gas introduced to the enclosureis consumed in the reduction reaction, and the rest is consumed in flamecurtain 13.

1. Method of continuously reducing oxides on the surface of metalcomprising continuously moving said metal through an enclosure having anentrance and an exit for said metal, heating at least the surface ofsaid metal in a heating zone near said entrance of said enclosure,introducing reducing gas to a cooling zone near said exit of saidenclosure, directing said reducing gas toward said surface of said metalin a vigorous and turbulent manner in a reducing zone in said enclosure,and burning unreacted reducing gas below a flue near said entrance forsaid metal to create a draft of said reducing gas in said enclosurecountercurrent to the movement of said metal.
 2. Method of claim 1including contacting at least one surface of said metal with elementalcarbon.
 3. Method of claim 1 wherein said exit for said metal includes afabric curtain.
 4. Method of claim 1 wherein inert gas is mixed withsaid reducing gas.
 5. Method of claim 1 wherein inert gas is mixed withsaid reducing gas in a ratio of 1:99 to 99:1.
 6. Method of claim 1wherein a portion of said reducing gas is heated before being introducedin said reducing zone
 7. Method of claim 1 whereby said reducing gas isheated within said enclosure before being directed toward said metal. 8.Method of claim 1 wherein said reducing gas comprises hydrogen. 9.Method of claim 1 wherein said reducing gas comprises carbon monoxide.10. Method of claim 1 wherein at least about 50% of said reducing gas isconsumed in reducing said oxides.
 11. Method of claim 1 wherein at leastabout 90% of said reducing gas is consumed in reducing said oxides. 12.Method of claim 1 wherein air is introduced near said entrance to saidenclosure to assist in burning said unreacted reducing gas.
 13. Methodof claim 1 wherein said metal is steel strip.
 14. Method of claim 1wherein said surface is heated to at least 900° F.
 15. Method ofcontinuously reducing oxides on the surface of metal comprisingcontinuously moving said metal through an enclosure having an entranceand an exit for said metal, heating at least the surface of said metalin a heating zone near said entrance of said enclosure, introducingreducing gas to a cooling zone near said exit of said enclosure,directing said reducing gas toward said surface of said metal in avigorous and turbulent manner in a reducing zone in said enclosure, andburning unreacted reducing gas.
 16. Method of claim 15 includingcontacting at least one surface of said metal with elemental carbon. 17.Method of claim 15 wherein said exit for said metal includes a fabriccurtain.
 18. Method of claim 15 wherein inert gas is mixed with saidreducing gas in a ratio of 1:99 to 99:1.
 19. Method of claim 15 wherebysaid reducing gas is heated before being directed toward said metal. 20.Method of claim 15 wherein said reducing gas comprises hydrogen. 21.Method of claim 15 wherein said reducing gas comprises carbon monoxide.22. Method of claim 15 wherein at least about 50% of said reducing gasis consumed in reducing said oxides.
 23. Method of claim 22 whereinsubstantially all of said reducing gas not consumed in reducing saidoxides is consumed in a flame curtain.
 24. Method of claim 15 wherein atleast about 90% of said reducing gas is consumed in reducing saidoxides.
 25. Method of claim 15 wherein air is introduced near saidentrance to said enclosure to assist in burning said unreacted reducinggas.
 26. Method of claim 25 wherein said air is directed at said metalto heat said metal by burning said unreacted reducing gas.
 27. Method ofclaim 15 wherein said surface is heated to at least 900° F.
 28. Methodof claim 15 wherein said burning of said unreacted reducing gas createsa draft of said reducing gas in said enclosure countercurrent to themovement of said metal.
 29. Method of claim 15 wherein said burning ofsaid unreacted reducing gas is in the form of a flame curtain. 30.Method of continuously reducing oxides in mill scale on the surface ofhot rolled steel strip comprising continuously moving said hot rolledsteel strip through an enclosure having an entrance and an exit for saidhot rolled steel strip, heating at least the surface of said hot rolledsteel strip in a heating zone near said entrance of said enclosure,introducing reducing gas to a cooling zone near said exit of saidenclosure, directing said reducing gas toward said surface of said hotrolled steel strip in a vigorous and turbulent manner in a reducing zonein said enclosure, and burning unreacted reducing gas.
 31. Method ofclaim 30 wherein said burning of said unreacted reducing gas isconducted near said entrance for said hot rolled steel strip and createsa draft of said reducing gas in said enclosure countercurrent to themovement of said hot rolled steel strip.
 32. Method of claim 30 whereinair is directed at said hot rolled steel strip to heat at least thesurface thereof by burning said unreacted reducing gas.
 33. Method ofclaim 30 wherein said burning of said unreacted reducing gas is in theform of a flame curtain.
 34. Method of claim 30 wherein said burning ofsaid unreacted reducing gas at least partially heats said hot rolledsteel strip.